A farm vehicle tyre has a tread with a toroidal base surface that extends about a central axis of rotation; and a number of lugs project radially from the base surface, and extend substantially crosswise to the travelling direction of the tyre.
At present, to produce a farm vehicle tyre, a tyre casing is prepared and wound onto a building drum; the tread belts are wound about the casing on the building drum, and a smooth, green-rubber blank tread is wound over the belts; and the casing, wound with the belts and the blank tread, is inserted inside a curing mold negatively reproducing the tread pattern. During the curing process, the rubber of the blank tread is heated and subjected to high pressure to adapt to the shape of the curing mold and so form the tread pattern.
The tread pattern includes the lugs, which are exceptionally large and, above all, project upwards considerably from the rest of the tread (even by as much as ten centimeters at the highest point). Forming the lugs in the curing mold therefore involves considerable rubber displacement inside the mold, i.e. a large amount of rubber must migrate from one part of the mold to another. This rubber displacement inside the curing mold, i.e. the large amount of rubber that has to be pressured into forming the lugs, prevents the formation of an evenly thick tread base. In other words, migration of a large amount of rubber inside the curing mold results in the formation of an ‘uneven’ tread base (i.e. differing widely in thickness) which may result in defects, rippling and unsightly flaws.
To prevent too thin a rubber thickness at the thinnest parts of the tread base, the average thickness of the tread base must be ‘oversized’ with respect to an optimum thickness that could be used if the tread base were of perfectly even thickness. In other words, to prevent too thin a rubber thickness at the thinnest parts of the tread base, the whole tread base must be made thicker using more rubber. Oversizing the average thickness of the tread base means more material (rubber) is required to produce the tyre (with no advantage whatsoever in terms of nominal tyre performance), thus increasing the manufacturing cost and weight of the tyre.
Moreover, because the size of the green-rubber tread cross section is calculated according to tyre size, the known production method described above requires a different green-rubber tread cross section for each tyre radius. In other words, the green-rubber tread cross section of a 420/85 R24 tyre, for example, cannot be used for a 420/85 R28, R30, R34 or R38 tyre.
To eliminate these drawbacks, it has been proposed to reduce the thickness of the blank tread wound about the casing (with the belts in between), and to apply green-rubber blocks to the areas of the blank tread where the lugs are to be formed. This reduces the amount of rubber that has to migrate from one part of the curing mold to another, by providing more rubber (in the form of rubber blocks) where it is needed (to form the lugs) right from the outset. Even using rubber blocks, however, the thickness of the tread base still remains significantly uneven, especially at the shoulders of the tyre where the lugs are larger. In other words, even using rubber blocks, the average thickness of the tread base must be ‘oversized’ with respect to an optimum thickness that could be used if the tread base were of perfectly even thickness.
U.S. Pat. No. 3,770,038A describes a method of producing a lugged tyre, whereby a green-rubber blank tread is produced; rubber blocks are applied to the areas of the blank tread where the lugs are to be formed; and the blank tread, together with the blocks, is cured in a curing mold.